Fix matrix overflow test case

This PR fixes up the test for successful non-overflow cases of f16 matrix vector multiplication. For example, for a mat2x2 vec2, this will now result in: MAX_FLOAT 0.5 + MAX_FLOAT 0.5 instead of the overflowing: MAX_FLOAT 1 + MAX_FLOAT * 1

Issue: #3765

Requirements for PR author:

[x] All missing test coverage is tracked with "TODO" or .unimplemented().
[x] New helpers are /** documented */ and new helper files are found in helper_index.txt.
[x] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
[x] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

[ ] Tests are properly located in the test tree.
[ ] Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
[ ] Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
[ ] Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

gpuweb / cts

Fix matrix overflow test case #3766